This document discusses surface modification of nanoparticles for biomedical applications. It describes how nanoparticles can be modified on their surface with various ligands to target specific cells and tissues. These ligands include antibodies, peptides, aptamers, and other molecules like folate. Aptamers and peptides offer advantages over antibodies as targeting ligands. The document provides examples of using various ligands like VEGF, folate, and transferrin to target receptors overexpressed on cancers and other diseases. It also discusses different conjugation chemistries used to attach ligands to the nanoparticle surface, like using succinimide, biotin-streptavidin, and thiol chemistry.